1. Field of the Invention
The present invention relates in general to the field of the fabrication of microstructures. More particularly, the present invention relates to a fabrication method of topographically modulated microstructures using pixel grayscale pattern homogenization with UV light.
2. Discussion of the Related Art
Most engineered microfluidic systems are restricted to two dimensions with a modulation of width and length. Moreover, “pseudo” 3D microfluidic devices are typically fabricated by stacking microfluidic layers and interconnecting them as an extension of the fabrication processes, i.e., they have “3D” connections, but flat topography. Modulating the topography of microfluidic channels is interesting because the height is typically the smallest dimension, and becomes a crucial parameter for a number of microfluidic physical phenomena through its relation to the Reynolds number (Re). In particular, the Reynolds number is proportional to the characteristic length of the system. Additionally, two other parameters are significant; the capillary pressure, Pc, which is inversely proportional to the radius of curvature of the liquid-air interface, and the flow resistance, R, which scales as a cubic power of the smallest dimension in rectangular microchannels. Local control over these three parameters in a microchannel will facilitate taking advantage of inertial effects on the flow (manipulation of Re), programming the wetting flow inside a chip (manipulation of Pc), or designing flow patterns (manipulation of Re) to produce different local shear stresses, chaotic advection, etc.
There are several fabrication technologies based on optical methods that have been developed to micro-sculpt 3D structures with great precision, but they are generally ill-suited for patterning large areas for microfluidic applications because of the large cost and time required. Alternatively, there are a number of photolithographic “grayscale” approaches where a “grayscale” mask with modulated UV transmittance is used to control the spatial exposure dose of UV light onto the photoresist material. After development, these can exhibit smooth surfaces of different heights. In general, many of the known techniques require a compromise between resolution, cost, and complexity of fabrication and design.
Fabrication using true grayscale masks (as seen at www.canyonmaterials.com) is particularly useful for the creation of high resolution 3D microstructures, but it is also expensive. Binary chrome masks are made of opaque and transparent pixels whose density determines the UV dose transmitted to produce 3D surfaces. Typically, these methods provide very good resolution at the expense of large amounts of processing that limit their practical use for patterning large areas. Additionally, a group at the University of Washington has demonstrated a fabrication method that uses ink flowing through microchannels to block UV light and pattern different heights correlated to the ink concentration in the aqueous solution. This method is useful for patterning large areas inexpensively and with virtually any tonality, but has limited flexibility to pattern arbitrarily sequential structures within those large areas. Alternative methods include the use of masks with opaque lines of different widths whose diffraction generates features of different heights, and the use of colored transparency masks with inks of different UV transmittance. In the first case, it is difficult to design a priori complex topographies because of the inherent complexity of diffraction. In the second case, the technique is limited by the available resolutions for colored transparencies. Other techniques are generally discussed in the article entitled “Using Pattern Homogenization of Binary Grayscale Masks to Fabricate Microfluidic Structures with 3D Topography,” Lab Chip, 2007, 7, 1567-1573, which was published in August of 2007 by the Royal Society of Chemistry, the entire contents of which are hereby expressly incorporated by reference into the present application.
What is needed therefore is a microfluidic device and fabrication method that differs from conventional binary grayscale fabrication technology in its principle and its form so as to allow for the patterning of large areas with three dimensional (3D) relief structures within a range of dimensional sizes and resolutions. What is also needed is the ability to fabricate microfluidic devices with three dimensional (3D) topography to promote the emergence of new microfluidic functionalities. Further, what are needed are photopolymerizable materials with different viscosity, contrast, and grayscale homogenization threshold to allow for the fabrication of microfluidic channels and other microstructures within smaller size ranges.